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design air conditioning system

Transcript of air conditioning (ch2)

  • 7

    Chapter Two

    Base case

  • 8

    (2-1)Air Conditioning Analysis

    Air conditioning system is designed to control the environment of occupied

    spaces, so the thermal comfort approached by occupants. So, according to seasonal

    variation of weather, air conditioning system has to cope with heating and cooling

    loads to achieve comfort ability.

    By conduction, convection and radiation sensible heat transmitted out of the

    building in winter, and transmitted to the building in summer, and by the same way;

    moisture can be transmitted in and out of the building, sensible and latent heat can be

    produced within the building according to occupants activity, lighting and machines

    also.

    The first step before any design process for any air conditioning system;

    engineers must carefully determine the amount of heat removal needed in summer

    season and the amount of heat to be produced in winter season . Before an air

    conditioning system can be designed, all these loads must be analyzed and summed

    up together with great care in order to select the most suitable equipment for the

    building systems.

  • 9

    (2-2) Overall Heat Transmission Factor

    RU

    1 (2-1)

    External wall heat transmission factor(U)[R]

    Figure (2-1): A schematic for external wall construction.

    Table (2-1):-external wall construction materials& thickness of material and

    thermal resistance.

    Material x(m) k(W/m.K) R(m2.K/W)

    cement plaster 0.025 1.2 0.02

    hollow brick 0.1 0.9 0.11

    air gap 0.1 0.28 0.36

    insulation 0.02 0.045 0.44

    concrete 0.15 1.75 0.09

    face stone 0.1 1.7 0.06

    outside air - - 0.029

    1nside air - - 0.12

    Floor heat transmission factor(U) UExternal = 0.815 (W/m

    2.K)

  • 10

    Figure (2-2): A schematic for Floor construction.

    Table (2-2): Floor construction materials & thickness of material and thermal

    resistance

    material x(m) k(W/m.K) R(m2.K/W)

    ceramic 0.02 1.05 0.02

    sand and gravel 0.1 0.7 0.14

    concrete baking 0.15 1.75 0.09

    hollow brick 0.18 0.9 0.2

    cement plaster 0.02 1.2 0.02

    1nside air - - 0.12

    1nside air - - 0.12

    UFloor= 1.42(W/m2.K)

  • 11

    Roof heat transmission factor(U)

    Figure (2-3): A schematic for roof construction. [1]

    Table (2-3): Roof construction materials& thickness of material and thermal

    resistance

    Material x(m) k(W/m.K) R(m2.K/W)

    asphalt water proving 0.02 0.7 0.029

    concrete baking 0.15 1.75 0.086

    Hollow brick 0.18 1.1 0.164

    cement plaster 0.02 1.2 0.017

    outside air - - 0.029

    1nside air - - 0.12

    URoof=2.26 (W/m2.K)

  • 12

    (2-3)Design Conditions and Desired Indoor Conditions

    Inside and Outside Design Condition:

    Inside design factors such as the following must be considered:

    Type and use of building

    Length or duration of occupancy

    Degree of activity of occupants

    Radiant heat source such as large lighting load or glass exposure.

    Outside design conditions and daily peak temperatures.

    Outside design conditions vary markedly with the locality. They are

    determined by averaging conditions, which occur over a number of years.

    Recommendations for Inside/Outside Design Conditions:

    For preparing any load calculation or in evaluating the equipment

    requirements, it is recommended to take into consideration the following

    points:-

    1- The desired inside design conditions are largely dependent on the load

    components and on the use and occupancy of the building.

    2- The most comfort applications, exact maintenance of space conditions is not

    required if the system can limit the variation to acceptable tolerances.

    3- Seldom do loads peak simultaneously in building.

    4- Off peak or partial load conditions may and frequently do produce more

    problems than the peak conditions. A successful system cannot be designed

    solely for a single set of load conditions.

    5- Economic operation of the system is to a large extent dependent upon the

    analysis of the load variation, kind of building, and building use.

  • 13

    (2-3-1)Design Conditions

    I. Outdoor Design Conditions:

    The building is located at Ramallah so the design conditions related to

    (Palestinian Guidelines for energy efficient building design) are as follows:

    a. winter (heating):

    Tdb = 2 oC

    =50%

    No humidity adjustment is needed.

    b. Summer (cooling):

    Tdb = 35 oC

    Twb = 24 oC

    And also no humidity adjustment is needed

    =60%

    Where:

    Tdb: the dry bulb temperature at Ramallah

    Twb: the wet bulb temperature at Ramallah

    : the relative humidity

    II. Indoor Design Conditions:

    The recommended design conditions for Office Buildings refer to (Palestinian

    Guidelines for energy efficient building design) were:-

    a. In winter (heating):

    Tdb= 23

    0C. (For patient room)

    Tdb= 21

    0C. (For CCU room)

    = 50%.

    b. In summer (cooling)

    Tdb= 23

    0C. (For patient room)

    Tdb= 21

    0C. (For Corridors)

    = 50%.

  • 14

    (2-4) Cooling Load

    Cooling load calculations deal with two types of heat gain:-

    a. Sensible Heat: heat that flows into or produced in a space will tend to

    cause a temperature rise in the space.

    b. Latent Heat: in the form of moisture which although it does not cause a

    temperature rise does change the condition of the air in the space resulting in a

    higher relative humidity.

    a)Sensible heat gains to the space include:

    a1. Heat transmission through the building structure as a result of

    conduction, convection and radiation.

    a2. Sensible heat brought in with the outside air, which is introduced, for

    ventilation.

    a3. Sensible heat produced by occupants.

    a4. Sensible heat produced in the space by lights, appliances, motors and

    the like.

    a5. Sensible heat to be extracted from materials or products brought into the

    space, (industrial air taming).

    b)Latent heat (moisture) gain may be classified as follows:

    b1. Latent heat from outside air ;( both that introduced for ventilation and

    that which infiltrates into the space).

    b2. Latent heat from occupants.

    b3. Latent heat from cooling, hot baths or other vaporization processes in the

    space.

    b4. Latent heat from products or materials brought into the space

    b5. Latent heat from equipment.

  • 15

    (2-4-1)Cooling Load Calculations

    The following features concern the Office building that we are going to

    calculate the cooling load for:

    Design Conditions:

    Outdoors design conditions:

    Tdb=35 0C.

    Twb = 240C.

    WO= 0.014

    Indoors design conditions:

    Tdb= 23

    0C. (For patient room)

    Tdb= 21

    0C. (For CCU room)

    WI=0.0085

    Figure (2-4):-Cooling design Conditions on the psychometric chart.

  • 16

    (2-4-2) Transmission Load

    A-External Walls:

    By using the general heat transfer equation which is:

    Q = UA (CLTD) (2 - 2) [2]

    Where:

    Q =Heat transmitted. (Watts).

    U =Overall heat transfer coefficient (W/m2.K).

    A=Area of exposed surface (m2).

    T=Temperature difference (To-Ti).

    But since all external walls in this building were exposed to the sun, so no

    need to this equation and instead using solar load equation which is described in the

    next section.

    B-Internal Walls:

    By using the general heat transfer equation (2-2)as the external walls, by

    noticing that T was equal to one or two degrees Celsius because of using the

    different temperatures for the offices, corridor and WCs.

    C-Windows:

    There are two type of heat gain through the windows which are by conduction

    and by solar; all windows were assumed to be made of single clear glass.

    1) Heat gain by conduction:

    From equation (2-2)

    2) Heat gain by solar

    Q solar heat gain = + (2-3)

    Where:

    SC: Shading coefficient

    CLF: Cooling load factor for glass

    SHGF: Solar heat gain factor for sunlit glass (W/m2)

    U: overall heat transfer coefficient (W/m2.K)

  • 17

    D- Doors:

    The external door was assumed to be made of single glass, so it was treated as

    windows. The Internal doors were made from wood.

    E- Roofs:

    Transmission load through the roof will be calculated by the solar load for the

    third (last) floor, while the roof for the other internal floors will not transmit heat

    since the floors are cooled, so there is no temperature difference just CCU room.

    F- Floors:

    Since the Office Building structure contains three floors, the first floor will

    transmit heat from the ground because it was assumed to be 12 0C.

    (2-4-3) Solar Load

    (a) Solar load through external walls and roof were calculated by using the following

    equation[R]:-

    adjCLTDUAq )( (2 4)[2]

    Where:

    U= Overall heat transfer coefficient (w/m2.oC).

    A= Area of the roof.

    CLTD=cooling load temperature difference. (From Appendix A).

    (CLTD) adj =CLTD + (25-Ti) + (Tav-